1. Field of the Invention
This invention relates to an automatic gauge control (AGC) for tandem rolling mills.
2. Description of the Prior Art
In processing strip material, such as tin for example, the product is successively reduced in thickness in tandem rolling mills, a high reduction in gauge being accomplished at the last stand of the series. The term high reduction here means a relatively large change in gauge of the strip material entering the last stand vis-a-vis the gauge of the material leaving the last stand. Such a process line is described in U.S. Pat. No. 3,740,983 for an "Automatic Gauge Control System for Tandem Rolling Mills", invented by Robert S. Peterson and John W. Cook. In the system described in this patent, the final output gauge is obtained by using AGC on the last stand to change the speed of the last stand, which then results in a change in tension in the strip material between the next to last, and the last stands. This interstand tension is then adjusted back to some preselected reference tension by displacement of the screw down setting for the last stand, thereby changing the roll gap setting of the last stand to that of the desired strip delivery gauge.
A special environment occurs in multistand cold mills where the finished product being rolled requires a roughened surface. Typically, such a situation arises where the final sheet product must be capable of supporting for example a coating of paint or zinc as is required in the production of galvanized sheet. In these situations the last stand of the tandem mill is customarily provided with sand blasted rolls. Contrary to usual tin mill operation, in mills such as this, very little strip reduction is accomplished by the sand blasted rolls of the last stand; typically this reduction is in the order of only 3%, so that screw down control for the last stand is not included as part of the AGC (the operator may be provided with optional manual means for screw down adjustment for the last stand, but this is not a part of the automatic gauge control (AGC)).
In any rolling operation where the objective is to control the gauge of the finished product, some kind of tension regulators are provided between all the stands. In the situation where screw down control is not permitted at the last stand, then tension is regulated by controlling the speed of the last stand. Such a system is described in U.S. Pat. No. 3,765,203 for "Automatic Gauge Control by Tension for Tandem Rolling Mills" invented by Robert S. Peterson. However, in using the teachings of this patent for rolling thin galvinized sheet (in the order of 10-25 mils thick), considerable difficulties were encountered, such as tearing of the galvanized sheet, with inevitable losses in production. Although it is axiomatic, it bears repeating, that when AGC is accomplished by tension control using speed adjustment between the last two stands, when there is a change in speed there is an interrelated change in tension. This requires a trade-off because there are limits to the permissible excursions for the tension parameter. If the mill were perfect, the operator would be given a tension reference to run the mill and that could produce the correct delivered gauge. Such a perfect mill does not exist in a real world, and hence, the operator is given permissible ranges for the changes in tension, for example +60%, -40%, i.e. the tension can increase 60% higher than the tension reference or decrease to 40% of the tension reference.
When rolling thick sheet, the increase in tension range can be tolerated fairly well, but the decrease in tension is troublesome. However, when rolling thin sheet i.e. in the order 10-25 mils, both the increases as well as the decreases in tension range are troublesome. When the tension increases too much, the thin sheet may be pulled apart. When the tension decreases there must be a trade off between roll force and tension. When tension is decreased, the roll force goes up. If the decrease in tension is of sufficient magnitude, the roll force will increase to the point where the rolls actually come down on the sheet product. This is known as a "pinch out", and again the sheet will be pulled apart.
If the teachings of U.S. Pat. No. 3,765,203 cited supra were applied to this situation, the AGC on the last stand saturates. In order to solve this the prior art teaches the utilization of range control to bring the AGC out of saturation; this can be accomplished by changing the speed of next to the last stand, but the required changes in speed are not necessarily in such direction as to provide the desired delivery gauge at the last stand, with the result that the delivered product tends to run off-gauge.
In this special rolling situation i.e. rolling thin galvanized sheet; the specifications for the finished rolled product, viz; a roughened surface, require that sand blasted rolls be utilized on the last stand of the tandem mill, with the result that prior art delivery (last stand) AGC using tension control by speed change, produces adverse effects which are cumulative, making this technique a non-workable solution. It has been empirically determined that a 3% reduction in product is all that can be accomplished at the last stand. If a greater swing in tension excursion is attempted, (resulting in greater than 3% reduction) then if the sheet does not tear first, then the sand blast coating is worn away or the rolls overheat. (This does not happen with the thicker product so wider tension excursion is possible.) The fact that sand blasted rolls are mandatory for this operation means that only a small or nominal reduction in gauge can be realized at the last stand. The concomitant effect is that this increases the tension excursion between the last two stands so that there is a practical limit on the permissible reduction in delivered gauge. Stated differently, even for only a 1% reduction in gauge at the last stand, there is a limit on the permissible swing in tension between the last two stands, because if the tension were permitted to fall too low, the roll force would go up, and the change in roll gap for the last stand could be large enough to cause the rolls to pinch--this would be catastrophic since it would result in a mill wreck. Generally speaking the screws on the last stand of a sheet mill (i.e. the last stand has sand blasted rolls) are not displaced.